1. Field of the Invention
This invention is directed to architectures for a transport network of a telecommunication system, and more particularly, to network architectures using transparent transport capabilities.
2. Background Art
The rapid evolution of the technology in recent years has made the optical fiber one of the most targeted transmission media, due mostly to the high transmission rates available and reduced error rates.
The Synchronous Digital Hierarchy (SDH) specifies a basic rate of 155.52 Mb/s, which is called synchronous transport module level-1 (STM-1). The smaller rate of 51.840 Mb/s is called synchronous transport signal level-1 (STS-1) and is the basic rate of the SONET (Synchronous Optical NETwork) version of SDH. Higher rates (STS-N, STS-Nc) are built from STS-1, and lower rates are subsets of this. An STS-N frame comprises an overhead (OH) field with administration, operation, maintenance and provisioning information, and a payload field with user information. The optical counterpart corresponding to an STS-N signal is called OC-N. To accommodate asynchronous signals from previous generations of transport equipment, North America (SONET) and Japan base their sub-STS-1 multiplexing hierarchies on the DS-1 rate of 1.544 Mb/s, while Europe (SDH) is based on the a 2.048 Mb/s rate. The level of synchronous multiplexing hierarchies where the schemes are common occurs at the European basic rate STM-1 and the North American rate STS-3. Thereafter, the three approaches multiplex these rates in multiple integers, all being compatible with the basic rates. While the present specification describes and illustrates signals of rate (or bandwidth) according to SONET networks, it is to be understood that the invention is applicable also to other synchronous networks.
It is well known that the topology of a synchronous optical network can have a linear point-to-point configuration or a ring configuration. A linear configuration protects the traffic on a working fiber (W) by using a protection fiber (P) which will carry the traffic if the working fiber is interrupted. A "1:1" system has an equal number of working and protection links, a "1:N" system has N working channels and one shared protection channel. Since the protection fiber is idle most of the time, extra-traffic (ET) of lower priority may be transmitted over the protection fiber.
The ring topology permits the network to also recover automatically from failures due to cable cuts and site failures. Currently, two types of SDH/SONET rings are used, namely unidirectional path switched rings (UPSR), and bidirectional line switched rings (BLSR). Both ring types support unidirectional and bidirectional connections.
The UPSR is typically used in the access network and therefore is built for lower rates, such as STS-3/STM-1, which are sufficient for access link demands. These rings are provided with bidirectional connections between nodes, yet the traffic flow is unidirectional. The signal is always present on both working and protection fibers, therefore, the protection fiber cannot be used to carry extra-traffic (ET).
The BLSR is typically used in the transport network, and therefore is built to operate at higher data rates, like STS-48/STM-16. For a four-fiber BLSR (4F-BLSR) the working and protection traffic flow on separate fibers, each for one direction. For a two-fiber BLSR (2F-BLSR), the fibers between adjacent nodes carry working traffic and also have protection capacity allocated within them. Bidirectional traffic between two adjacent nodes takes place in the working time-slots, and protection traffic is inserted in the protection time-slots. Since for a BLSR configuration the protection timeslots are only used during a protection switch, they can be used for lower priority ET. Due to the working timeslots reuse capability, a BLSR always provides the optimum use of bandwidth for a given traffic pattern. However, an automatic protection switching (APS) protocol is necessary.
A traffic node is defined as the transmission equipment deployed at a site. In practical configurations, a site may comprise equipment belonging to different networks co-located in the same operation center. Such scenarios are common in big cities. There are many benefits to supporting large bandwidths on a single piece of equipment. Reducing the amount of equipment at a site simplifies the network management and also means fewer trips to a location for equipment repairs and replacement. The key benefit is lower equipment cost.
Telecommunications network providers are feeling the pressure of upgrading the equipment to the level of the latest technologies, as users demand ever more capacity. That factor, along with the reality of fiber congestion in the network, is causing providers to search for a solution that will increase capacity without forcing them to deploy additional fibers.
For an existing linear system that is experiencing fiber exhaust on a given span, the traditional solution is to replace the relevant equipment to obtain a higher line rate system. However, for a ring configuration, the line rate of the entire ring must be upgraded even if only one span is short of fiber. It is thus easy to understand why some network providers are asking for other options.
The add/drop multiplexer combines various STS-N input streams onto an optical fiber channel. Transparent transport is defined herein as the ability to provide continuity of all payloads and associated overhead bytes necessary to maintain a lower bit rate linear or ring system through a higher bit rate midsection, while reducing the required number of fibers interconnecting the sites. The lower bit rate linear or ring system operates as if it were directly connected without the higher bit rate midsection. Description of a transparent multiplexer, referenced as "TMux", is provided in the U.S. patent application Ser. No. 08/847526, filed on Apr. 24, 1997 by Martin et al., assigned to Northern Telecom Limited and entitled "Transparent Multiplexer/Demultiplexer". A method for transparently transporting higher rates signals over a mid-span is disclosed in the U.S. patent application Ser. No. 08/847529, filed on Apr. 24, 1997 by Martin et al., assigned to Northern Telecom Limited and entitled "Transparent transport".
In summary, transparency in this specification implies that the bytes of the trib overhead are manipulated by the TMuxs such as to not require altering the provisioning of the existing systems, to maintain their protection switching, maintenance signalling, section/line/path performance monitoring, and to provide sufficient information for fault isolation. For example, if the trib rate is OC-48 and the midspan rate is OC-192, one solution possible is to carry the working (W) channels for all OC-48 trib systems on the OC-192 (W) channel, and the trib protection (P) channels over the OC-192 P-channel, without OC-192 protection switching enabled (defined in the above patents as the "nailed up" OC-192 option). In this arrangement, a failure of the OC-192 W-channel would trigger a span switch of all trib systems.
Eight OC-48 lines, or thirty OC-12/OC-3 lines can be consolidated over the high rate midspan, as detailed in the above mentioned patent applications. Bidirectional couplers may be used to further reduce the fiber count on the high rate span, i.e. from four to two fibers. It is to be noted that the bandwidth efficiency provided, 20 Gb/s bidirectional over two fibers, is accomplished without the transponders and tight tolerance transmitters and dense WDM couplers necessary in the equivalent WDM solution.
The invention is not limited to OC-3/OC-12/OC-48 trib signals carried by an OC-192 supercarrier, but it is also adaptable to other bit rates, in accordance with the hardware and software evolution of transport networks. Also, the invention is not limited to equipping of only identical trib rates, it is possible to carry transparently trib signals of different trib rates over the high rate span. The input tribs described in this invention have the same rate for an easier understanding of the general concept. In addition, the invention is not limited to SONET signals, and it can be applied to other synchronous transport technologies.